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Classification and Properties of Copper Alloys

Abstract:

Copper and copper alloys are some of the most versatile engineering materials available. The combination of physical properties such as strength, conductivity, corrosion resistance, machinability and ductility make copper suitable for a wide range of applications. These properties can be further enhanced with variations in composition and manufacturing methods.

Copper is the oldest metal used by man, its use dates back to prehistoric times. Copper has been mined for more than 10,000 years with a copper pendant found in current day Iraq being dated to 8700BC.

By 5000BC copper was being smelted from simple copper oxides. The first high-strength copper alloy gave its name to an era, the Bronze Age, which followed the Copper (Chalcolithic) Age more than 4,000 years ago. Early bronzes consisted of copper and tin, and the “phosphorus bronzes” that we now use for electrical purposes (which contain around 5% tin and very little phosphorus) have been called the direct descendents of these primitive alloys. Phosphorus bronzes cannot be regarded as having high conductivity, since their conductivity is less than 10% that of pure copper.

Copper is found as native metal and in the minerals cuprite, malachite, azurite, chalcopyrite and bornite. It is also often a by-product of silver production. Sulfides, oxides and carbonates are the most important ores.

Copper and copper alloys are some of the most versatile engineering materials available. The combination of physical properties such as strength, conductivity, corrosion resistance, machinability and ductility make copper suitable for a wide range of applications. These properties can be further enhanced with variations in composition and manufacturing methods.

Selection of Copper Alloys

The primary selection criteria for copper and copper alloys include:

Electrical conductivity: copper has the highest conductivity of the engineering metals. Silver or other elements may be added to increase strength, softening resistance or other properties without major loss of conductivity.

Thermal conductivity: this property is similar to electrical conductivity. Alloys of copper may be used for this property, where good corrosion resistance compensates for loss of conductivity with increased alloying.

Color and appearance: many of the copper alloys have a distinctive color, which may change as the object weathers. For most of alloys it is easy to prepare and maintain the surface to a high standard, even in adverse corrosion conditions. Many of the alloys are used in decorative applications, either in their native form or after metal plating. The alloys have specific colors, ranging from the salmon pink of copper through yellow, gold and green to dark bronze in the weathered condition. Atmospheric exposure can produce a green or bronze surface, and prepatinated alloys are available in some product forms.

Applications of Copper

Copper and copper alloys can be used in an extraordinary range of applications. Some of these applications include:

Power transmission lines

Architectural applications

Cooking utensils

Spark plugs

Electrical wiring, cables and busbars

High conductivity wires

Electrodes

Heat exchangers and refrigeration tubing

Plumbing

Water-cooled copper crucibles.

The largest end use for copper is in the building industry. Within the building industry the use of copper-based materials is broad. Construction industry related applications for copper include:

Roofing

Cladding

Rainwater systems

Heating systems

Water pipes and fittings

Oil and gas lines

Electrical wiring.

Types of Copper Alloys

Commercially pure coppers are very soft and ductile, containing up to about 0.7% total impurities. These materials are used for their electrical and thermal conductivity, corrosion resistance, appearance and color, and ease of working. They have the highest conductivity of the engineering metals and are very ductile and easy to braze, and generally to weld. Typical applications include electrical wiring and fittings, busbars, heat exchangers, roofs, wall cladding, tubes for water, air and process equipment.

High copper alloys contain small amounts of various alloying elements such as beryllium, chromium, zirconium, tin, silver, sulfur or iron. These elements modify one or more of the basic properties of copper, such as strength, creep resistance, machinability or weldability. Most of the uses are similar to those given above for coppers, but the conditions of application are more extreme.

Brasses are copper zinc alloys containing up to about 45% zinc, with possibly small additions of lead for machinability, and tin for strength. Copper zinc alloys are single phase up to about 37% zinc in the wrought condition. The single phase alloys have excellent ductility, and are often used in the cold worked condition for better strength. Alloys with more than about 37% zinc are dual phase, and have even higher strength, but limited ductility at room temperature compared to the single phase alloys. The dual phase brasses are usually cast or hot worked.

Bronzes are alloys of copper with tin, plus at least one of phosphorus, aluminum, silicon, manganese and nickel. These alloys can achieve high strengths, combined with good corrosion resistance. They are used for springs and fixtures, metal forming dies, bearings, bushes, terminals, contacts and connectors, architectural fittings and features. The use of cast bronze for statuary is well known.

Copper nickel are alloys of copper with nickel, with a small amount of iron and sometimes other minor alloying additions such as chromium or tin. The alloys have outstanding corrosion resistance in waters, and are used extensively in sea water applications such as heat exchangers, condensers, pumps and piping systems, sheathing for boat hulls.

Nickel silvers contain 55–65% copper alloyed with nickel and zinc, and sometimes an addition of lead to promote machinability. These alloys get their misleading name from their appearance, which is similar to pure silver, although they contain no addition of silver. They are used for jewelry and name plates and as a base for silver plate (EPNS), as springs, fasteners, coins, keys and camera parts.

The corrosion resistance of copper alloys comes from the formation of adherent films on the material surface. These films are relatively impervious to corrosion therefore protecting the base metal from further attack.

Ductility can be restored by annealing. This can be done either by a specific annealing process or by incidental annealing through welding or brazing procedures.

There are four common ways to harden (strengthen) copper. A fifth, spinodal composition, is currently used commercially only in certain copper-nickel-tin alloys. Combinations of strengthening mechanisms are often used to provide higher mechanical properties in high-copper alloys.

Strain Hardening. The application of cold work, usually by rolling or drawing, hardens copper and copper alloys. Strength, hardness and springiness increase, while ductility decreases. Conductivity is reduced to a small extent, normally not to the extent that it hinders use of the alloys in electrical products. The effect of cold work can be removed by annealing, in which case full conductivity returns. Strain hardening is the only strengthening mechanism that can be used with pure copper.

Solid-Solution Hardening. Alloying elements that remain dissolved in solidified copper strengthen the lattice structure. If the addition is within the limit of the element’s solid solubility, no secondary phases form, and the appearance under the microscope is similar to that of pure copper.

All dissolved additions to copper reduce electrical conductivity, making the balance between strengthening gained and conductivity lost necessarily a compromise. The extent of this effect on conductivity varies widely from element to element. Cadmium additions, for example, affect conductivity least, while others, such as phosphorus, tin and zinc, are more detrimental. In any case, cold working can be used to increase strength beyond the limits of solid solution hardening, and the two strengthening mechanisms are frequently used in combination.

Precipitation Hardening. Some alloying elements exhibit higher solubility in solid copper when hot than when cold. This means they can be dissolved by solution treatment (solution annealing) at high temperatures, around 950–1000°C, and then removed from solution by a precipitation (or "aging") treatment at a lower temperature, commonly around 1200°F (650°C). This practice produces a fine precipitate throughout the metal that strengthens the matrix without spoiling the conductivity. In fact, conductivity improves as precipitates drop out of solution. Beryllium, chromium and zirconium are common examples of this type of addition. Combinations of nickel with silicon or phosphorus are also useful.

Dispersion Strengthening. Particles of insoluble or even inert materials can also be finely distributed within a copper matrix by metallurgical, mechanical or chemical means, i.e., without having to resort to heat treatment. Being insoluble, the particles have little effect on electrical conductivity.

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